The invention relates to a flexible hollow shaft to be received in a curved shank of a medical instrument,
A surgical instrument having a flexible hollow shaft of this kind is known from EP 0 986 989 A1.
The flexibility of the hollow shaft means that the instrument or its shank can have a curved design. The flexible hollow shaft is received in the curved shank and its proximal end is connected to a drive mechanism which rotates the hollow shaft in the curved shank. At the distal end, the hollow shaft is provided with a tool, for example with a cutting edge or with a milling head. The distal end of the hollow shaft comes to lie freely rotatably in the distal end area of the curved instrument shank.
In the aforementioned EP 0 986 989 A1, the tool is designed as a cutting edge that is used to remove tissue.
DE 101 07 156 A1 discloses a medical instrument in which the tool is designed as a milling head that protrudes from the distal end of the curved instrument shank.
The wall of the hollow shaft is provided with a large number of cuttings, by which the flexibility is achieved. These openings consist, for example, of cuttings which are made in a meandering formation in the wall and which extend along a helical line. In this way, with a curved and rotating hollow shaft, it is possible for the cuttings on the outer face of the curvature to widen slightly.
In practical application, tensile forces and/or compressive forces act on the tool.
In the case of a milling head, for example as used in operations on the frontal sinuses, compressive forces occur when the milling head encounters a bone site and the operating surgeon pushes the instrument sharply against this bone site. Tensile forces may occur if the milling head has worked its way into the bone and an attempt is made to withdraw the instrument, such that the milling head catches on the bone.
Similar compressive loads/tensile loads occur in the case of cutting edges that become caught on tissue or cartilage.
In practical use, it has now been found that the flexible hollow shaft is able to take up tensile and/or compressive loads by virtue of its construction, i.e. the hollow shaft can be slightly extended or compressed. In these states, however, the hollow shaft turns no longer round with the curvature of the curved shank, but bulges slightly outward and strikes against the inside face of the curved shank, as a result of which undesired vibrations occur that make handling of the medical instrument difficult.
It is therefore object of the present invention to remedy this situation and to provide measures that prevent such undesired vibration or outward deflection of the hollow shaft.